A number may be made by multiplying two or more other numbers together. The
numbers that are multiplied together are called factors of the final number. All
numbers have a factor of one since one multiplied by any number equals that number.
All numbers can be divided by themselves to produce the number one. Therefore, we normally
ignore one and the number itself as useful factors.

The number fifteen can be divided into two factors which are three and five.

The number twelve could be divided into two factors which are 6 and 2. Six could
be divided into two further factors of 2 and 3. Therefore the factors of twelve are
2, 2, and 3.

If twelve was first divided into the factors 3 and 4, the four could be divided
into factors of 2 and 2. Therefore the factors of twelve are still 2, 2, and 3.

There are several clues to help determine factors.

Any even number has a factor of two

Any number ending in 5 has a factor of five

Any number above 0 that ends with 0 (such as 10, 30, 1200) has factors of two and five.

To determine factors see if one of the above rules apply (ends in 5, 0 or an even number).
If none of the rules apply, there still may be factors of 3 or 7 or some other number.